Method of case hardening



Patented Jan. 11, 1944 UNITED STATES PATENT OFFICE 2,339,223 METHOD or case momma Donald A. Holt, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware N Drawing. Application May 23, 1940,

' Serial No. 336,885 I 12 Claims. (01. 148-15) material; by the gas hardening'method wherein the articles are heated in an atmosphere of carbonaceous gas; and by heating in certain wellrecognized types of fused salt baths containing cyanide or other cyanogen compound. Usually the treatment of steel in a fused salt bath containing cyanide causes both carburization and nitriding. A certain well-recognized type of cyanide case hardening bath known as a carburizing bath tends to introduce appreciable amounts of carbon into the steel and relatively small amounts of nitrogen. This type of case hardening bath is well adapted to the production of relatively deep carburized cases and may be used for that purpose in place of the conventional pack hardening and gas hardening methods.

In producing ahigh carbon case by any of the above-mentioned methods and especially when it is desired to produce a deep case for most purposes, it is essential that the carbon concentration in the case should not substantially exceed the eutectoid carbon concentration. It is often difflcult to avoid introduction of excessive carbon in the case hardening operation and this is especially true when treating alloy'steel such as nickel steels where the eutectoid carbon concentration is appreciably lower than in plain carbon steel. If the carbon concentration in the case greatly exceeds the eutectoid carbon concentration, the desired physical properties for most purposes are not obtained. For example, when this concentration is exceeded in carburizing steel by the pack hardening method, the resulting grain structure of the case ordinarily is relatively coarse, especially when a deep case is produced by a relatively long time treatment at a high temperature. In such cases when the case is quenched after hardening, it has been observed that free, cementite has precipitated in the grain boundaries of the case producing a surface structure having undesirable physical properties.

introduction of carbon beyondthe eutectoid concentration as mentioned above. Hence, for the majority of purposes, very careful control is required to produce a case of the desired physical properties when using a fused salt bath for the purpose.

An object of the present invention is to provide an improved method for case hardening steel articles whereby a deep carbon case having good physical properties may be obtained. A further object is to produce a deep carbon case characterized by small grain size and substantial absence of cementite in the grain boundaries after heat treatment. Still other objects will be apparent from the following description of my invention.

In accordance with my invention, I am-able to overcome the difliculties mentioned above by following the carburizing treatment with a nitriding treatment in a fused salt bath containing cyanide. I have discovered that as a result of the nitriding treatment, the physical properties of the case are materially improved especially as regards ductility and impact strengths and that the formation of free cementite in the grain boundaries of th case on quenching is substan tially eliminated. In practicing my invention, I prefer to utilize for the nitriding operation a fused salt bath containing alkali metal cyanide or other suitable cyanogen compound operated at a temperature not lower than 750 C. and preferably not higher than 850 C.

Altemately, I may use an ammonia. g'as nit rid must contain at least 1 volume, but not more In carburizing steel with a fused salt cyanide than about 5 volumes of the carburizing gas.

The mixture may be diluted withan inert gas,

for xample, with an equal volume of nitrogen.

In a preferred method of practicing my invention, I first carburize the steel-article in a carburizing type of fused salt cyanide case hardening bath such as that described hereinafter. In order to obtain efiicient operation, I carry out this carburizing treatment at a relatively high temperature, for example, not lower than 900 C. and preferably around 950 C. for the period of cult to control th p ration so as to avoid the 5: time required toproduce the depth of case desired.

This treatment may be continued, for example, as long as 50 to 100 hours toproduce an exceptionally deep case. After the carburizlng treatment, the steel may be either quenched or aircooled; the method and degree of cooling is immaterial. The article then is heated in the fused salt nitriding bath, for example, a salt bath containing or more of alkali metal cyanide at .,a temperature suitable for nitriding, for example, within the temperature range of 750-850 C. This nitriding treatment must be carried out for a suificient time to permit fairly deep penetration of nitrogen into the case. While this time of treatment will vary considerably depending upon the depth of case produced by the preceding carburization treatment, in most cases treating a nickel alloy steel containing about from the nitriding bath, it is quenched, prefer ably in oil, either from the nitriding temperature or from some other temperature, depending upon the nature of the steel treated and the physical properties desired. .If desired, the article then maybe given further conventional heating treatments such as tempering or the liketo further modify the physical properties of the article.

Various carburizing case hardening baths suitablefor practicing the invention as above described are known and described in the literature concerning case hardening. Such baths ordinarily contain, in addition to alkali metal cyanide or other suitable cyanogen compound and suitable inert salts'such as .alkali metal halides and carbonates, an appreciable quantity of an alkaline earth metal salt. The quantity of alkaline earth metal salt varies in different baths of this type and depends chiefly upon the kind of alkaline earth metal salt selected. Likewise, the amount of cyanide present also will vary depending upon the selection of alkaline earth metalhalide. Often such baths may contain two or more of the alkaline earth metal salts. Various suitable compositions of this type are known-and a number of suitable salt m xtures are available on the market. To

. salt mixture which contains 6 to 12% of alkali metal cyanide, and to 60% of barium salts.

0.24% of carbon for 24 to 48 hours, the fully hardenable case depth was found to be about 0.120 to 0.140 inch, this being the depth at which the carbon concentration was 0.50%. By treating the same steel for 16 to 32 hours, the corresponding case depths were 0.092 to 0.130 inch. Afterthe above-described case hardening treatment, the steel article then is reheated at a lower temperature in a fused salt nitriding bath which contains from 10 to 100% by weight of alkali cyanide, for example, a bath containing about'l0 to 75% by weight of sodium cyanide. This reheating bath is of the type which produces a high nitrogen case and therefore does not contain excessive amounts of alkaline earth metal salt. Preferably, the bath is free from any alkaline earth metal salt. The steel article is maintained in the nitriding bath at a temperature of 750 to 850 0., preferably about 830' C. for about 1 to 3 hours. The article then is quenched in oil, preferably from the temperature of the nitriding bath. .The treated article then may be subjected to further heat treatments as may be desirable, for example it may be tempered at a temperature of 150 to 350 C. I

I have found that by means of this method of case hardening, a deep carbon case can be ob tained which has .an unusually high degree of ductility. The case has also-been found to have unusually fine grain structure and there is no evidence of cementite precipitation in the grain boundaries. My process is especially welladapted for treating alloy steels having a low eutectoid.

' carbon concentration, for example alloy steels those skilled in case hardening, the various prowhich contain significant amounts of one or more of the following elements: nickel, chromi um, vanadium, molybdenum, manganese, silicon, titanium and tungsten.

I claim:

1. The case hardening process whichcomprises subjecting a steel article containing less than the eutectoid carbon concentration to a carburizing treatment in a fused salt case hardening bath containing alkali metal cyanideand alkaline earth metal salt at a temperature of approximately 950 C. for a period of about 16 to 100 hours-to produce a fully hardenable case-depth of at least 0.09 inch, thereafter subjecting the so carburized article to a nitriding treatment in a fused salt bath containing 10 to 75% by weight of alkali metal cyanide for aperiod of -1 to 5 hours at a temperature of about 830 C. and quenching the reheated article from the temperature of the hitriding bath, to produce a carburized case of such After fusion and during operation, such fused bath may contain, e. g.,, around 10% of cyanide and 50% of barium chloride.

As an example of practicing my invention, a steel article first is heated in a cyanide case hardening bath of the type above described at a temperature of 900 to 970 C. for a period of ment for a period of about 20 to 50 hours. to 7| a carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the aforesaid nitriding temperature.

2. The case hardening process which comprises subjecting a steel article containing less than the.

eutectoid carbon concentration to a carburizing treatmentin a fused salt bath containing 6 to 12% by weight of alkali metal cyanide and 30 to 60% by weight of barium chloride at a temperature of 900 to 970 C. for a period of about 16 to 100 hours to produce a fully hardenable casedepth of at least 0.09 inch and thereafter subjecting the so carburized article to a nitriding treatment in a fused salt bath containing 10 to by weight of alkalimetal cyanide for a period of 1 to 5 hours at a temperature of about.

830 C. and quenching, to produce a carburized of such carbon content that there is no sub-- eutectoid carbon stantial formation of free cementite in thegrain boundaries on quenching from the aforesaid nitriding temperature.

3. The case hardening procms which comprises subjecting a steel article containing less than the concentration to a deep case carburizing treatment at a temperature not less than 900? C. for a period of about 16 to 100 hours to produce a fully hardenable case-depth of at least 0.09 inch and thereafter subjecting the so carburized article to a nitriding treatment at a temperature of 750 to 850 C. for at least one hour in .a nitriding medium selected from the group consisting of (1) mixtures of ammonia and carburizing gas containing 1 to 5 volumes of carburizing as for each volume of ammonia and (2) fused salt baths containing cyanogen compounds, to produce a carburized case of such carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the aforesaid nitriding temperature.

4. The case hardening process which comprises burizing treatment in a fused salt carburizing bath at a temperature not lower than 900 C. for a length of time suflicient to produce a fully hardenable case-depth of at least 0.09 inch, and thereafter subjecting the so carburized article to a nitriding treatment in a fused salt bath containing a cyanogen compound at a temperature of 750-850 C. for at least one hour, to produce a carburized case of such carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the subjecting a steel article containingless than the eutectoid carbon concentration to a case carburizing treatment at a temperature not less than 900 C. for a length of time sufiicient to produce a fully hardenable case-depthof at least 0.09 inch and thereafter subjecting the so carburized article to a nitriding treatment at a temperature of '750-850 C. for at least one hour in a nitriding medium selected from the group consisting of (1) mixtures of ammonia. and carburizing gas containing 1 to 5 volumes of carburizing gas for each volume of ammonia and (2) fused salt ba'tlm containing cyanogen compounds, to produce a carburized case of such carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the aforesaid nitriding temperature. I

5. The case. hardening process which comprises subjecting a steel article containing less than the eutectoid carbon concentration to a casecarburizing treatment at a temperature not less than 900 C. for a length of time suflicient to produce a fully hardenable case-depth of at least 0.09 inch and thereafter subjecting the socarburized article to a nitriding treatmentin a fused salt bath containing a cyanogen compound at a temperature of ISO-850 C. for at least one hour,

to produce a carburized case of such carbon con- 1 article to anitriding treatment in a fused salt bath containing out less than about 10% by weight of alkali metal cyanide at a temperature of 750-850 C. for at least on'e hour and quenching said article, to produce a carburized case of such carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the aforesaid nitriding temperature.

'7. The case hardening process which comprises subjecting a steel article containing less than the eutectoid carbon concentration to a case carquenching deep case carburizing aforesaid nitriding temperature.

8. The case hardening process which comprises subjecting a steel article containing less than the eutectoid carbon concentration to a deep case carburizing treatment in a fused salt carburizing bath containing an alkali metal cyanide and an alkaline earth metal salt at a temperture not lower than 900 C. for a length of time sufllcient to produce a fully hardenable case-depth of at least 0.09 inc thereafter subjecting the so carburized article to a nitriding treatment in a fused salt bath containing not less than about 10% by weight of alkali metal cyanide at a temperature of 750-850? C. for at least one hour and quenching said article, to produce a carburized case of such carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the aforesaid nitriding temperature. I

9. The case hardening process which comprises subjecting an alloy steel article containing less than the eutectoid carbon concentration to a case carburizing treatment at a temperature not lower than 900 C. for a length of time sufficient to produce a fully hardenable case-depth of at least 0.09 inch, and thereafter subjecting the so carburized article to a nitriding treatment in a fused salt bath containing a cyanogen compound at a temperature of 750 850 C. for at least one hour, to produce a carburized case of such carbon content that there is'no substantial formation of free cementite in the grain boundaries on ature.

10. The case hardening process which comprises subjecting a nickel-containing alloy steel article containing less than the eutectoid carbon concentration to a deep case carburizing treatment at a temperature not lower than 900 C. for a length of time suiiicient to produce a fully hardenable case-depth of at least 0.09 inch thereafter subjecting the so carburized article to a nitriding tretament in a fused salt bath containing not less than about 10% by weight of alkali metal cyanide at a temperature of '750-850 C. for at least one hour,and quenching said article, to produce a carburized case of such carbon content that there is no substantial formation of free cementite in the grain boundaries on quenching from the aforesaid nitriding temperature.

11. The case hardening process which come prises subjecting a steel article containing less than the eutectoid carbon concentration to a treatment at a temperature not lower than 900 C. for a length of time suilicient'to produce a fully hardenable casedepth of at least 0.09 inch and thereafter subjecting the so carburized article to a nitriding treatment at a temperature of 750-850 C. for at least one hour in a mixture of ammonia and a carburizing gas containing 1 to 5 volumes of carburizing gas to each volume of ammonia, to produce a carburized case of suchcarbon content that there is no substantial formation of free from the aforesaid nitriding tempercementite in the grain boundaries on quenching from the aforesaid nitriding temperature.

12. The case hardening process which comprises subjecting a steel article containing less than the eutectoid carbon concentration to acarbunzing treatment in a fused salt carburizing bath containing a cyanogen compound and alkaline earthmetal salt at a temperature of 900 to 970 C. for a length of time sufficient to produce a. fully hardenable case-depth of at least 0.09

Y inch, thereafter subjecting the so carburized ar- 10 triding temperature.

' DONALD A. HOLT.

about 10% by weight 

